Why doesn't my parent process see the child's output until it exits?

Question

Consider the following script:

use IO::File;
$| = 1;
my ($handle, $pid) = myPipe();
if ($pid == 0) {
  print \"$$\";
  sleep 5;
  exit;
}

print \"child: \".         


        

Answer 1

On some (most?) systems, pipes use I/O buffering by default. Put a

$handle->autoflush(1);

statement in your myPipe function.

But even when buffering is turned off, Perl still doesn't flush except after a newline. So you may also want your child process to include a newline in the output.

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Update: Testing your code (Cygwin, perl 5.10.0, YMMV), I see the issue is a lack of the newline in the child output, not whether autoflush is explicitly turned on when the pipe is created.